[llvm-branch-commits] [CodeGen] Limit number of analyzed predecessors (PR #142584)

[Alexis Engelke via llvm-branch-commits]

================
@@ -1030,6 +1036,11 @@ bool MachineBlockPlacement::isTrellis(
   SmallPtrSet<const MachineBasicBlock *, 8> SeenPreds;
 
   for (MachineBasicBlock *Succ : ViableSuccs) {
+    // Compile-time optimization: runtime is quadratic in the number of
+    // predecessors. For such uncommon cases, exit early.
+    if (Succ->pred_size() > PredecessorLimit)
----------------
aengelke wrote:

Consider the code from the test generator below. From my understanding, `buildChain` will iterate over all basic blocks of the chain and call `selectBestSuccessor` for each of them, which will in turn call `isTrellis` for every block. `isTrellis` will look at the predecessors of all successors, in particular, it will look at all predecessors of the `merge` block, which are all the other blocks => for almost every block, the code looks at almost all other blocks.

Test generator, try n=40000:
```python
import sys

n = int(sys.argv[1])
print("declare void @exit(i32)")
print("declare i1 @cond(i32)")
print("define i32 @f(i32 %v, i32 %v0) {")
for i in range(n):
    print(f'b{i}:')
    print(f'  %v{i+1} = add i32 %v{i}, %v')
    print(f'  %c{i} = call i1 @cond(i32 %v{i+1})')
    print(f'  br i1 %c{i}, label %merge, label %b{i+1}')
print(f'b{n}:')
print(f'  ret i32 %v{n}')
print('merge:')
print('  call void @exit(i32 1)')
print('  unreachable')
print('}')
```
```console
# Without this change
$ python3 many-preds3.test 40000 | /usr/bin/time ./llvm-build/bin/llc -filetype=obj -o /dev/null -O1
15.93user 0.17system 0:16.18elapsed 99%CPU (0avgtext+0avgdata 457748maxresident)k
0inputs+0outputs (0major+99524minor)pagefaults 0swaps
# With this change
$ python3 many-preds3.test 40000 | /usr/bin/time ./llvm-build/bin/llc -filetype=obj -o /dev/null -O1
8.82user 0.19system 0:09.10elapsed 99%CPU (0avgtext+0avgdata 457240maxresident)k
0inputs+0outputs (0major+99425minor)pagefaults 0swaps
```

https://github.com/llvm/llvm-project/pull/142584